1. Field of the Invention
This invention relates to a polyolefin composition containing talc.
2. Description of the Prior Art
Heretofore, large quantities of food packaging containers have been produced by vacuum or pressure forming of highly rigid plastic sheets such as polyvinyl chloride or high impact polystyrene. The use of materials of this kind, however, presents some difficulties. Polyvinyl chloride is suspected to be carcinogenic because of the monomer remaining therein, or damages incinerating furnaces as a result of evolving hydrogen chloride when burned therein. High impact polystyrene is likely to be partly dissolved out when in contact with fatty foods such as butter or margarine. Accordingly, in order to avoid pollution, the development of pollution free containers has been desired for food packaging and other purposes.
As is well known, polyolefins are highly safe materials for packaging foodstuffs, but suffer from the defects that they have low rigidity, produce extreme heat at the time of burning and their vacuum or pressure formability is poor.
We have made investigations in order to increase the rigidity of polyolefins, reduce the amount of heat they generate at the time of incineration and to improve their vacuum or pressure formability by incorporating inorganic fillers therein. In order, however, to incoporate inorganic fillers in polyolefins it is necessary either to use special polyolefins or to modify the surface of the inorganic fillers to increase their affinity with the polyolefins.
Inorganic filler containing polyolefin compositions which emerged from these investigations, however, were not satisfactory with respect to whiteness, vacuum or pressure formability and odor. In particular, in spite of the fact that talc is most suitable from the standpoint of acid resistance, cost, and ability to render the composition rigid, the incorporation of talc in polyolefins results in compositions having a poor color and a strong odor, which are not suitable for food packaging containers requiring cleanliness.
As to the use of talc incorporated into polyolefins, U.S. Pat. No. 3,553,158 discloses a composition comprising a thermoplastic hydrocarbon polymer, talc, a talc-deactivator and antioxidant stabilizers. Talc of a high metal content not only deteriorates the whiteness of a composition comprising a polyolefin such as polypropylene and a stabilizer but also markedly deteriorates the heat stability of the product. Therefore, in order to prevent such deterioration, a specific additive (e.g., a talc-deactivator) other than the stabilizer must be added to the composition, but where such a specific additive is added, it adversely influences vacuum or pressure formability, odor and the like of the composition. Specifically, where a composition of this kind is used to produce food packaging containers, the sheet formed from the composition must be odor free, must have good whiteness and must be easily fabricated into food packaging containers by vacuum or pressure forming techniques. In order to achieve the above objects, it is necessary to reduce the number and the amount of additives present to as low as possible.
The present invention overcomes the above-mentioned conventional defects by using talc having a metal content not higher than a specific level.